To meet the demand for wireless data traffic having increased since deployment of 4th generation (4G) communication systems, efforts have been made to develop an improved 5th generation (5G) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘Beyond 4G Network’ or a ‘Post Long Term Evolution (LTE) System’.
The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 28 GHz or 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems.
In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud Radio Access Networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP), reception-end interference cancellation and the like.
In the 5G system, Hybrid frequency shift keying (FSK) and quadrature amplitude modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
The next generation wireless system is expected to address different use cases having quite different requirements in terms of data rate, latency, reliability, mobility etc. However, the next generation wireless system is expected that the design of an air-interface of the next generation would be flexible enough to serve a set of User Equipment's (UEs) having quite different capabilities depending on the use case and market segment the UE cater service to an end customer. Few example use cases, the next generation wireless system is expected to address an enhanced Mobile Broadband (eMBB), massive Machine Type Communication (m-MTC), ultra-reliable low latency communication (URLL) etc. The eMBB requirements like tens of Gbps data rate, low latency, high mobility so on and so forth address the market segment representing the conventional wireless broadband subscribers needing internet connectivity everywhere, all the time and on the go.
The m-MTC requirements like very high connection density, infrequent data transmission, very long battery life, low mobility address so on and so forth address the market segment representing the Internet of Things (IoT)/Internet of Everything (IoE) envisioning connectivity of billions of devices. The URLL requirements like very low latency, very high reliability and variable mobility so on and so forth address the market segment representing the Industrial automation application, vehicle-to-vehicle/vehicle-to-infrastructure communication foreseen as one of the enabler for autonomous cars.
In the conventional systems, the UE in a connected state communicates with an Enhanced Node B (eNB). A radio protocol stack for communication between the UE and the eNB comprises of Packet Data Convergence Protocol (PDCP), Radio link control (RLC), a Medium Access Control (MAC) and Physical (PHY) sub layers. One or more data radio bearers (DRBs) are established between the UE and the eNB for exchanging user plane packets. Each DRB is associated with one PDCP entity and one or more RLC entities. Each DRB is associated with a logical channel in the MAC sub layer. There is one MAC entity in the UE for the eNB.
In the conventional systems, the main services and functions of the MAC sublayer include: mapping between logical channels and transport channels, Multiplexing/de-multiplexing of MAC SDUs belonging to one or different logical channels into/from transport blocks (TB) delivered to/from the physical layer on the transport channels, scheduling information reporting, error correction through HARQ, priority handling between the logical channels of one UE, priority handling between the UEs by means of dynamic scheduling, transport format selection and padding.
The main services and functions of the RLC sublayer include: transfer of upper layer PDUs, error correction through ARQ (only for Acknowledged Mode (AM) data transfer), concatenation, segmentation and reassembly of RLC SDUs (only for Un-acknowledgement Mode (UM) and AM data transfer), re-segmentation of the RLC data PDUs (only for the AM data transfer), reordering of the RLC data PDUs (only for the UM and AM data transfer), duplicate detection (only for the UM and AM data transfer), protocol error detection (only for the AM data transfer), the RLC SDU discard (only for the UM and AM data transfer), and RLC re-establishment.
Functions of the RLC sub layer are performed by the RLC entities. The RLC entity can be configured to perform the data transfer in one of the following three modes: Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM). Consequently, the RLC entity is categorized as a TM RLC entity, an UM RLC entity and an AM RLC entity depending on the mode of data transfer that the RLC entity is configured to provide. The TM RLC entity is configured either as a transmitting TM RLC entity or a receiving TM RLC entity. The transmitting TM RLC entity receives RLC SDUs from an upper layer (i.e. PDCP) and sends RLC PDUs to its peer receiving the TM RLC entity via lower layers (i.e. MAC). The receiving TM RLC entity delivers the RLC SDUs to the upper layer (i.e. PDCP) and receives the RLC PDUs from its peer transmitting the TM RLC entity via the lower layers (i.e. MAC).
Further, the UM RLC entity is configured either as a transmitting UM RLC entity or a receiving UM RLC entity. The transmitting UM RLC entity receives the RLC SDUs from the upper layer and sends the RLC PDUs to its peer receiving UM RLC entity via the lower layers. The receiving UM RLC entity delivers the RLC SDUs to the upper layer and receives the RLC PDUs from its peer transmitting the UM RLC entity via the lower layers. The AM RLC entity consists of a transmitting side and a receiving side. The transmitting side of the AM RLC entity receives the RLC SDUs from the upper layer and sends the RLC PDUs to its peer AM RLC entity via the lower layers. The receiving side of the AM RLC entity delivers the RLC SDUs to the upper layer and receives the RLC PDUs from its peer AM RLC entity via the lower layers.
The main services and functions of the PDCP sublayer for the user plane include: header compression and decompression: ROHC only, transfer of user data, in-sequence delivery of the upper layer PDUs at PDCP re-establishment procedure for RLC AM, For split bearers in the DC (only support for the RLC AM): PDCP PDU routing for transmission and PDCP PDU reordering for reception, duplicate detection of the lower layer SDUs at the PDCP re-establishment procedure for the RLC AM, retransmission of the PDCP SDUs at handover and, for split bearers in DC, of PDCP PDUs at PDCP data-recovery procedure, for RLC AM, ciphering and deciphering, and timer-based SDU discard in an uplink (UL). Functions of the PDCP sub layer are performed by the PDCP entities. Each PDCP entity carries the data of one radio bearer. Due to the UE mobility, the UE may handover from one eNB to another eNB. In DC due to UE mobility, the UE may handover from one MeNB to another MeNB or SCG change from one SeNB to another SeNB. The eNB may support multiple cells and the UE may also handover from one cell to another cell of same eNB. After the handover, the user plane protocols for the DRBs configured with the RLC layer in the AM mode are handled as follows in a legacy system: a PDCP SN is maintained on a bearer basis; a source eNB informs the target eNB about the next DL PDCP SN to allocate to a packet which does not have a PDCP sequence number yet (either from the source eNB or from a serving gateway); For security synchronisation, a Hyper Frame Number (HFN) is also maintained; the source eNB provides to the target one reference HFN for the UL and one for the DL i.e., HFN and corresponding SN; Security keys are refreshed; the UE sends PDCP status report to the target eNB if the PDCP is configured by the target eNB. The configuration to send status report is per bearer; the target eNB may send the PDCP status report to the UE and the UE does not need to wait to resume UL transmission; the UE re-transmits in the target eNB or the target cell, all uplink PDCP SDUs starting from the first PDCP SDU following the last consecutively confirmed PDCP SDU i.e., the oldest PDCP SDU that has not been acknowledged at the RLC in the source, excluding the PDCP SDUs of which the reception is acknowledged through the PDCP SN based reporting by the target eNB. The target eNB re-transmits and prioritizes all downlink PDCP SDUs forwarded by the source eNB (i.e., the target eNB should send data with the PDCP SNs from X2 before sending data from S1), with the exception of the PDCP SDUs of which the reception is acknowledged through the PDCP SN based reporting by the UE; the ROHC is reset; and the RLC/MAC is reset. The PDCP PDUs stored in the PDU reordering buffer are deciphered and decompressed and kept in the PDCP, associated with COUNT.
Alternately, after the handover the user plane protocols for DRBs configured with RLC in the UM mode are handled as follows in legacy system: the PDCP SN is reset; the HFN is reset; the security keys are refreshed; No PDCP status report is transmitted; No PDCP SDUs are retransmitted in the target eNB; The UE PDCP entity does not attempt to retransmit any PDCP SDU in the target cell for which transmission had been completed in the source cell. Instead UE PDCP entity starts the transmission with other PDCP SDUs; ROHC is reset; and RLC/MAC is reset.
In case of intra eNB handover from one cell to another, a PDCP entity location is unchanged (i.e., remain in same eNB), so that the security key refresh is unnecessary in a legacy procedure and leads to delay in data transmission/reception in the target cell as new keys needs to be synchronized between the UE and the target cell and data which are already encrypted in the source cell needs to be re-encrypted using the new key.
In the next generation communication system, new radio access network architecture comprising of centralized units (CUs) and distributed units (DUs) is being considered. There can be one or more DUs under one CU. The radio protocol stack or functions for communication may be split between the CU and the DUs in various manners. For example, in one option PDCP layer/functions are located in the CU and the RLC/MAC/PHY functions/layers are located in the DU. In another option, the PDCP/RLC layers/functions are located in the CU and the MAC/PHY layers/functions are located in the DU. Similarly there can be other options to split functionality between the CU and the DU. In such a radio access network architecture, due to the UE mobility, the UE may move from one DU to another DU within same CU or the UE may move from one DU to another DU in different CU. In another scenario, the UE may detect a Radio Link Failure (RLF) on the serving DU and then it switches to the target DU within same CU or different CU. For these scenarios i.e., inter CU DU change or intra CU DU change handling of the user plane protocols/functions is not defined for both RLF and non RLF cases. The simplest approach would be to follow the same operation as defined for handover in the legacy system. However, this is not efficient.
The above information is presented as background information only to help the reader to understand the present invention. Applicants have made no determination and make no assertion as to whether any of the above might be applicable as Prior Art with regard to the present application.